Traditionally, soap has been used as a skin cleanser. While soap is low in cost, easy to manufacture and lathers well, it is also very harsh on skin.
In order to alleviate the harshness of soap, synthetic bars have been used in which much of the soap is replaced with milder surfactants, e.g., acyl isethionates. Patents relating to the use of acyl isethionate and soap, therefore, are known (see U.S. Pat. No. 2.894,912 to Geitz).
It is also known to make bars which are even milder by replacing soap, isethionate or fatty acid (used primarily as structurant) with very mild surfactants such as amphoteric surfactants. Normally, however, it is extremely difficult to successfully and economically process bars containing both mild anionics and amphoterics (e.g., betaine).
U.S. Pat. No. 5,372,751 to Rys-Cicciari et al. does teach bar compositions comprising anionic (e.g., acyl isethionate) and betaine. The reference notes at several points that soap is preferably absent (column 6, lines 60-61; column 9, line 47) and this is confirmed by examples where soap is never used in amounts greater than 2%. While the reference suggests this is done for reasons of mildness, applicants have also previously never been able to process amounts of betaine above 1% at these low levels of soap.
Unexpectedly, applicants have found that when minimal levels of fatty acid soap (e.g., 3% and up) are used in bars comprising an anionic surfactant system, much greater levels of amphoteric (2% and up) can be readily processed than previous possible.
Applicants have further discovered that when the total content of saturated soap to unsaturated soap is greater than 1:1, process benefits (e.g., rate of plodding) are enhanced yet further. At the same time, the ability to successfully process more betaine allows introduction of much greater mildness benefit.